The present disclosure generally relates to masks for coded aperture systems and methods of using the masks, and more particularly, to masks that maximize the efficiency for stand-off radiation detection and imaging systems.
Given the desire of terrorist organizations to obtain nuclear weapons or other radiological weapons such as “dirty” bombs, serious efforts are being made to assess this nation's vulnerabilities and to enhance the nation's security. Potential areas of vulnerability can include, for example, seaports, airports, urban areas, borders, stadiums, points of interest, and the like. In U.S. seaports, for example, an average of about 16,000 cargo containers arrive by ship every day, any one of which could be used to conceal fissile material or an assembled nuclear device. Furthermore, once in the country, the nuclear material could travel virtually anywhere in the country with little to no detection capability.
A currently prevailing model for addressing such threats associated with potentially reactive material could be characterized as a customs-based approach, where radiation detection systems are integrated into the existing customs infrastructure at ports and border crossings. Once the containers leave the customs area, additional screening methods are required to investigate potential threats once within the county's borders.
Several methods exist for detecting nuclear material once within the nation's borders. These systems largely consist of devices which can detect radiation but neither definitely locate the source or discriminate between naturally occurring sources of radiation and genuine threats. The devices include small pager-size devices and larger Geiger-counter based detectors. These devices rely on measuring a local increase in the detection of gamma-rays to determine the presence of radioactive material. Because they do not perform any imaging or energy discrimination, they often indicate false-positive threats potentially leading to ignoring true threats. To passively detect and locate radioactive material that could be used in potential terrorism threats domestically, several technologies have been considered. Attenuating collimators to achieve the radioactive localization suffer from low efficiencies and can have significant weight issues to attenuate high energy gamma-rays. Compton cameras can be used due to their localization abilities, but their inherent inefficiencies at low radiation energies, high cost, and high system complexity make them undesirable for such applications.
Systems for detecting radioactive material can employ coded aperture imaging. Coded aperture imaging provides a means for improving the spatial resolution, sensitivity, and signal-to-noise ratio (SNR) of images formed by x-ray or gamma ray radiation. In contrast to these other systems, for instance, the coded aperture camera is characterized by high sensitivity, while simultaneously achieving exceptional spatial resolution in the reconstructed image.
Sources of such high energy electromagnetic radiation (i.e., X-ray, gamma-ray or neutron sources) are generally imaged by coded aperture arrays onto a detector which has detector elements arranged in a pattern of rows and columns. Imaging techniques based on coded apertures have been successfully applied by the astrophysics community, and are now being developed for national security purposes.
Current coded aperture systems utilize a mask with multiple, specially-arranged pinholes or transmission regions to increase the overall photon transmission, and hence the sensitivity, of the imaging camera. In operation, radiation from the object to be imaged is projected through the coded aperture mask and onto a position-sensitive detector. The coded aperture mask contains a number of discrete, specially arranged elements that are either opaque or transparent to the incident photons. Every point source within the detector's field of view casts a shadow of the aperture pattern onto the detector plane. Each shadow is displaced an amount commensurate with the angular displacement of the point source from the system's central axis. The sum total of the radiation pattern recorded by the detector constitutes the “coded” data, which usually bears no resemblance to the actual source. The raw signal from the detector does not reflect a directly recognizable image, but instead represents the signal from the object that has been modulated or encoded by the particular aperture pattern. This recorded signal can then be digitally or optically processed to extract a reconstructed image of the object. In addition, the data can be further processed to extract spectroscopic information to determine the type of source that emitted the radiation.
The aperture mask is typically a one- or two-dimensional planar array of the occluding and transmission regions. The mask, particularly the occluding (i.e. opaque) regions, can be made of an attenuating material. Examples of attenuating materials suitable for aperture masks can include tungsten, lead, and the like. Prior art FIG. 1, is a simplified illustration of a standard aperture mask 10 in front of a position sensitive detector (PSD) 12. The PSD 12 detects radiation emitted from a radiation source 16. The angle subtended by the mask 10 and the PSD 12 determines the field of view, i.e., the fully encoded region. Dashed lines 14 are shown to represent the field of view. As can be seen, the larger the mask, the wider the field of view. The mask, therefore, is often made larger to trade off the overall detector size for a wider field of view. This can be undesirable, however, for radiation detection systems that are meant to be portable since it means a relatively small fraction of the overall detection system size will be sensitive to the incident radiation. Moreover, a typical aperture mask, such as the mask 10, can reduce the efficiency of the detector to less than 50 percent even if half of its area is transparent to the radiation. In other words, the mask is one of the main determining factors in the size and efficiency of the detection system when it is being used to identify the location and specific isotopes of the source of the radiation.
To reiterate, a standard coded aperture system with a typical coded mask can be used, but suffers from a modest radiation sensitive area and limited efficiency. Such a standard coded aperture system, therefore, may not be desirable for a standoff radioactive imaging system application, particularly wherein it is desirable for the system to be easily portable and highly efficient.